1. Field of the Invention
This disclosure is generally related to a passive optical network (PON). More specifically, this disclosure is related to transporting a radio frequency (RF) signal over a PON.
2. Background Art
In order to keep pace with increasing Internet traffic, network operators have widely deployed optical fibers and optical transmission equipment, substantially increasing the capacity of backbone networks. A corresponding increase in access network capacity is also needed to meet the increasing bandwidth demand of end users for triple play services, including Internet protocol (IP) video, high-speed data, and packet voice. Even with broadband solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks still presents a severe bottleneck in delivering large bandwidth to end users.
Among different competing technologies, passive optical networks (PONs) are one of the best candidates for next-generation access networks. With the large bandwidth of optical fibers, PONs can accommodate broadband voice, data, and video traffic simultaneously. Such integrated service is difficult to provide with DSL or CM technology. Furthermore, PONs can be built with existing protocols, such as Ethernet and ATM, which facilitate interoperability between PONs and other network equipment.
Typically, PONs are used in the “first mile” of the network, which provides connectivity between the service provider's central offices and the premises of the customers. The “first mile” is generally a logical point-to-multipoint network, where a central office serves a number of customers. For example, a PON can adopt a tree topology, wherein one trunk fiber couples the central office to a passive optical splitter/combiner. Through a number of branch fibers, the passive optical splitter/combiner divides and distributes downstream optical signals to customers and combines upstream optical signals from customers (see FIG. 1). Note that other topologies are also possible, including ring and mesh topologies.
Transmissions within a PON are typically performed between an optical line terminal (OLT) and optical network units (ONUs). The OLT controls channel connection, management, and maintenance, and generally resides in the central office. The OLT provides an interface between the PON and a metro backbone, which can be an external network belonging to, for example, an Internet service provider (ISP) or a local exchange carrier. For EPON, the interface is an Ethernet interface. The ONU terminates the PON and presents the native service interfaces to the end users; the ONU can reside in the customer premises and couple to the customer's network through customer-premises equipment (CPE).
FIG. 1 illustrates a passive optical network including a central office and a number of customers coupled through optical fibers and a passive optical splitter (prior art). A passive optical splitter 102 and optical fibers couple the customers to a central office 101. Multiple splitters can also be cascaded to provide the desired split ratio and a greater geographical coverage. Passive optical splitter 102 can reside near end-user locations to minimize the initial fiber deployment costs. Central office 101 can couple to an external network 103, such as a metropolitan area network operated by an ISP. Although FIG. 1 illustrates a tree topology, a PON can also be based on other topologies, such as a logical ring or a logical bus. Note that, although in this disclosure many examples are based on EPONs, embodiments of the present invention are not limited to EPONs and can be applied to a variety of PONs, such as ATM PONs (APONs), Broadband PONs (BPQNs), gigabit PONs (GPONs), and wavelength division multiplexing (WDM) PONs.
In order to compete with other service providers, multiple system operators (MSOs), which traditionally provide cable television (CATV) services to their subscribers, are also developing new solutions that can leverage their networks and subscriber base to create profitable and differentiated services. Nowadays, many MSOs implement EPON solutions that can deliver triple-play services to subscribers. However, these EPON solutions need to be able to support existing MSO service-delivery architecture and equipment. For example, it is desirable to allow the subscribers to use the same set-top box (STB) used in cable settings to transmit and receive analog signals via the EPON. Such STBs often use an out of band (OOB) analog signal for interactive services, including video on demand (VOD) and Pay TV (PTV) programs. Society of Cable Telecommunications Engineers (SCTE) 55-1 and 55-2 are two standard specifications for the transmission of OOB signals.